Fix nameservice provider auto-generated IDs that may overlap.

When the ID autogenerator was implemented in the nameservice provider base, it
had a small bug: it would always use the GID of the entry to find the largest
number, regardless of it looking for a GID or a UID.

This means that if you did not use the modern convention of per-user primary
groups, and your highest UID was larger than your highest primary GID, you
would happily generate overlapping IDs.

While the method itself isn't awesome, it is broadly enough used that we
shouldn't get rid of it entirely. This fixes it to correctly determine the
largest UID for UID search, and GID for GID search, with less metaprogramming.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>